The proceeding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.
The probe coil of a pulsed NQR detector having a radio frequency (RF) transmitter and an RF receiver is a device providing interaction between the radio frequency (RF) field of the RF transmitter and a target substance irradiated by the RF field, as well as between the RF field response from the target substance and the RF receiver of the NQR detector, pursuant to being irradiated by the RF field. This interaction is implemented as a rule by a coil that constitutes a part of the resonant circuit of the probe coil, which coil circumscribes a scanned volume that can be permeated by the RF field and which can receive an RF field responsive to a target substance irradiated by the RF field within the scanned volume.
The sensitivity of the NQR detector is determined to a large extent by the efficiency of this interaction, with the degree of homogeneity of the magnetic component of the RF field within the scanned volume being designated as its main characteristic. The homogeneity of the magnetic component of an RF field is mathematically described by the gradient of the function of the magnetic component.
The detection of substances in large volumes places special requirements on the sensitivity of the NQR detector and, consequently, on the homogeneity of the RF field inside the volume circumscribed by the coil.
For example, when searching for substances (such as explosives or narcotics) inside a package or luggage item, the amount of the substance to be detected will usually occupy a relatively insignificant volume, located somewhere within the item to be scanned. The natural requirement therefore is for the response from the substance to be independent of the position of the substance within the volume circumscribed by the coil.
When using a multi-pulse sequence, such as that described in our co-pending International Patent Application PCT/AU00/01214, the level of the registered signal depends on the average value of the amplitude of the magnetic component of the RF field within the period of the sequence, determined at the location of the sample as described in the paper by S S Kim, J R P Jayacody and R A Marino, “Experimental investigations of the Strong Off-resonant Comb (SORC) pulse sequence in NQR”, Z Naturforsch. 47a 415–420 (1992). Therefore, the requirement stated above for the response of the substance to be independent of the position of the substance within the scanned volume is associated with the need for a homogeneous RF field in the total volume circumscribed by the coil.
To create a homogeneous RF field at the substance location, U.S. Pat. No. 5,168,224 (Maruizumi et al) suggests the use of a paired NQR detector coil arrangement consisting of two flat identical spirals placed in parallel spaced apart relationship on the same axis, with the target substance placed in the space between them. Such a coil arrangement was studied in detail within the article by Rudakov et al, “A System of Coils for Detecting Signals of Nuclear Quadrupole Resonance”, Instrument and Experimental Techniques 41 398–400 (1998). A disadvantage of this system of coils is the low homogeneity of the distribution of the RF field along the radius of the spiral, with especially strong variations observed near the surfaces of each of the spirals.
In U.S. Pat. No. 5,457,385 (Sydney and Shaw) two arrays of parallel spaced apart coils are described, the coils being arranged into sets of opposed coil pairs with one coil of a set disposed in one array and the other coil of the same set disposed in the other array, opposite to each other. In addition to simple single annular coil arrangements, it is suggested to use a more complex coil arrangement for one of the coils to comprise a pair of square and flat spirally wound coils, electrically connected in parallel, and disposed in parallel juxtaposed relationship, instead of a single angular coil arrangement. Thus the resultant coil consists of two windings etched onto opposite sides of an insulating board each winding being in the shape of a flat band of constant width and being radially displaced to each other so that one winding is juxtaposed between the gap of the other winding. The width of the band and the gaps between the turns of the band are equal so that the resultant plane of the coil is continuously and alternately covered by the windings, except for a central rectangular region that is not occupied by either of the coils. The distance between the parallel planes of the windings, is provided by the thickness of the insulating board, and is so small that it can be assumed to be zero when studying the RF fields generated by the coil. The radial displacement of the turns of each winding corresponds to the width of the band, which serves to exclude variations in the RF field amplitude near to the coil windings. However, both windings are situated inside a region forming an annular belt, which circumscribes the central rectangular region that is devoid of coil windings. The disadvantage of such a system of coils is the fact that inside the volume circumscribed by this system, particularly near the surface of the spiral coils, the distribution of the RF field depends on the coordinates of the points, within which the amplitude of the RF field is determined in space. When moving from the external periphery of the coil to its centre close to its surface, the amplitude of the RF field first increases over the coil windings in the annular belt, and then drops sharply over the central rectangular region. Another disadvantage of this system is the necessity to use two windings for each of the spirals to exclude variations of the RF field amplitude near the turns of the coil in the annular belt.
The disadvantage of the coils suggested in patents U.S. Pat. No. 5,168,224 (Maruizumi et al) and U.S. Pat. No. 5,457,385 is that to create a sufficiently homogeneous RF field inside the scanned object, the geometrical dimensions of the coils must considerably exceed the geometrical dimensions of the scanned object. The use of such coils as compared with coils that have dimensions similar to the scanned objects, results in an increase in the RF power of the NQR spectrometer necessary to achieve the required RF field intensity, and results in a decreased filling factor of the coil and therefore a reduced sensitivity (to the NQR response).